Upper tropospheric CO and O.sub.3 budget during the Asian Summer Monsoon

During the Asian Summer Monsoon, the circulation in the Upper Troposphere-Lower Stratosphere (UTLS) is dominated by the Asian Monsoon Anticyclone (AMA). Pollutants convectively uplifted to the upper troposphere are trapped within this anticyclonic circulation that extends from the Pacific Ocean to the eastern Mediterranean basin. Among the uplifted pollutants are ozone (O.sub.3) and its precursors, such as carbon monoxide (CO) and nitrogen oxides (NO.sub.x). Many studies based on global modelisation and satellite data have documented the source regions and transport pathways of primary pollutants (CO, HCN) into the AMA. Here, we aim to quantify the O.sub.3 budget by taking into consideration anthropogenic and natural sources. We first use CO and O.sub.3 data from the Metop-A/IASI sensor to document their tropospheric distributions over Asia, taking advantage of the useful information they provide on the vertical dimension. These satellite data are used together with MOZAIC/IAGOS tropospheric profiles recorded in India to validate the distributions simulated by the global GEOS-Chem chemistry transport model. Over the Asian region, UTLS monthly CO and O.sub.3 distributions from IASI and GEOS-Chem display the same large-scale features. UTLS CO columns from GEOS-Chem are in agreement with IASI, with a low bias of 11 ± 9% and a correlation coefficient of 0.70. For O.sub.3, the model underestimates IASI UTLS columns over Asia by 14 ± 26% but the correlation between both is high (0.94). GEOS-Chem is further used to quantify the CO and O.sub.3 budget through sensitivity simulations. For CO, these simulations confirm that South-Asian anthropogenic emissions have a more important impact on enhanced concentrations within the AMA (∼25 ppbv) than East-Asian emissions (∼10 ppbv). The correlation between enhanced emissions over the Indo--gangetic--Plain and monsoon deep convection is responsible for this larger impact. Consistently, South-Asian anthropogenic NO.sub.x emissions also play a larger role in producing O.sub.3 within the AMA (∼8 ppbv) than East-Asian emissions (∼5 ppbv) but Asian lightning produced NO.sub.x are responsible for the largest O.sub.3 production (10--14 ppbv). Stratosphere to Troposphere Exchanges (STE) are also important in transporting O.sub.3 in the upper part of the AMA.